Automatic transmission control



May 3, 1938. P. o. FARNHAM AUTOMATIC TRANSMISSION CONTROL 2 snets-sheet 1 Filed April '7, 1932 D l l- Hlj P. O. FARNHAM Filed April '7, 1932 2 Sheets-Sheet 2 gym/wm: @d w.

AUTOMATIC TRANSMISSION CONTROL May 3, 1938.

Patented May 3, 1938 UNITED STATES PATENT OFFICE AUTOMATIC TRANSMISSION CONTROL Delaware Application April 7, 1932, Serial No. 603,849

19 Claims.

This invention relates to automatic control circuits of the type employed in wave transmission systems, particularly in radio receivers, and more especially to methods of and circuit arrangements for suppressing all output from the transmission system when the magnitude of the received Wave or signal falls below a predetermined level.

The invention will be described in connection with the reception of radio broadcast signals but it will be apparent that the invention may be applied to other types of signal transmission systems. Due to the high gain of receivers of the automatic gain control type, the noise output is undesirably high when the receiver is tuned off a carrier wave or when, through fading, the received signal energy falls to a low value. A large part of this noise is fluctuation noise arising directly in or from the tubes and circuits at the beginning of the amplifying system, and peak voltages at the detector of the order of four or five volts may result from this cause. In addition to receiver noise, there is an often greater component due to interference arising outside of the receiver, such as from electrical disturbances on power lines. For satisfactory reception, it Would be desirable to effect operation of an automatic gain control receiver in such manner that the audio output will be completely suppressed Whenever the received signal energy falls below that n level at which the noise output is significant in comparison to the signal output.

A negative bias on the anode of a diode detector will suppress all audio output until the peak input voltage on the diode exceeds the bias voltage but, unfortunately, the continued pres- 'ence of the bias voltage required for best noise suppression may be so high as to produce some distortion in the audio output.

Objects of the present invention are to provide o improved methods of and circuit arrangements for controlling the transmission through an eleclil trical wave transmission system. Further objects are to provide methods of and circuit arrangements for automatically controlling the transmis- 5v sion in such manner that a substantially constant and satisfactory output is obtained for all input voltages above a critical value, and the output is completely suppressed for all inputs below that critical value which will produce a satisfactory output. Further specific objects are to provide a transmission system in two sections of which the gain is automatically varied in opposite directions by the incoming signal. More specifically, an object is to provide a transmission system, for .example a radio receiver of the high gain type,

which includes an automatic gain control for maintaining a substantially constant output for all values of signal input above a critical value and an automatic transmission control for suppressing all output for radio input voltages below that level at which the noise output becomes significant in comparison to the signal output.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawings in which:

Fig. 1 is aschematic diagram illustrative of the novel method of controlling the transmission through a radio receiver;

Fig. 2 is a fragmentary circuit diagram of one embodiment of the invention, and

Fig. 3 is a curve sheet illustrating the relationship between audio output and signal input for several adjustments of the circuit of Fig. 2.

In the schematic diagram, Fig. 1, the several stages of a complete radio receiver are represented by the units A to H which are arranged between the antenna or input system and the reproducer l. These stages may be combinations of radio frequency amplifiers, a detector and audio amplifiers, or certain of the stages may be frequency changers and intermediate frequency ampliers such as are employed in superheterodyne receivers. The design of the several stages and the general method of adding an automatic gain control system, indicated as an auxiliary unit J, to receivers of various types are well known.

The present invention relates particularly to the provision, in such a system of the automatic gain or transmission control unit K, and more 3 particularly, its relationship to the automatic gain control unit J. v

The gain control unit J comprises an automatic bias-developing agency taking its input from the point l in the transmission system and furnishing a control upon the radio frequency amplication of the system preceding its input point I through the channels a, b and c. Since the input to control unit J is in the form of a modulated radio frequency voltage, and the output is usually in the form of a direct current bias suitable for application to the vacuum tubes of the preceding stages, it is apparent that the operation of the unit J involves the process of rectification though it may, in addition, include amplification preceding or following the rectification.

In accordance with the invention, a second bias-developing agency K is connected to the transmission system at some point 3 which may,v in one special case, be at the voltage level point l 55 but which, in the general case, may precede, coincide with, or follow the point I. The output from the unit K is employed through one or more channels e, f, to control the transmission of the receiver system subsequent to point I, and in such manner that, for radio frequency voltage levels E1 at point I less than a critical Value, the unit K does not permit transmission through the remainder of the system to the audio output at reproducer I. In general, a suppression of trans-` mission at some point subsequent to the automatic gain control connection I is to be preferred .to a suppression at a preceding point and, for insuring a sharper cut-o, it is desirable to have the control unit K permit free` transmission of signals, with increasing radio frequency input, before the normal gain control of the unit J is initiated.

In the particular circuit arrangement shown in Fig. 2, the two control units J and K are connected to the transmission system at the same point I, which point coincides with the output voltage across the tuned circuit of unit E of the receiver system as shown diagrammatically in Fig. 1. Dio-de rectification is employed and, for economy and simplicity, the rectifier elements of .the units F, J and K are enclosed in a common envelope, i. e., the multiple element tube I functions as three diode rectiiiers in parallel.

The radio frequency voltage E1 developed across the input impedance il by the amplifier action of the preceding stages is impressed on tube Ill by connecting the high signal potential terminal of impedance H to the cathode C of the tube, and grounding the low potential terminal through a condenser I2. Tube I9 may be a special tube having three anodes but a commercial screen grid tube, with a control grid G1 and screen grid G2 may be employed. The elements will be designated as grids in the following description but it will be apparent that the elements function independently as anodes for rectification.

Grid G1 and cathode C constitute the audio frequency rectifier or demodulator of the system, plate P and cathode C constitute the rectier of the automatic gain control unit J, and grid G2 and. cathode C constitute the rectifier of the transmission control unit K.

The direct current circuit of the gain control rectifier is completed by the output resistance R1 which is connected between plate P and ground, and a connection I3 from the low potential terminal of the input impedance Il to a point of positive potential with respect to-grouncl, such as point Y on the Voltage divider resistance I4 of a rectifier-filter system (not shown) which is energized, as is common practice, from a source of alternating current. With this arrangement, the cathode C is more positive than plate P by a steady delay-bias voltage Ep, andno rectification takes place until the peak input voltage E1 eX- ceeds the bias voltage Ep. When the input voltage E1 exceeds the bias voltage Ep, the rectified voltage developed across resistance R1 is returned, through a filter comprising resistance i5 and condenser I6, to a preceding stage or stages as a gaincontrol voltage.

The direct current circuit of the control unit K includes the resistance R2 which is connected between the grid G2. and a point S, preferably adjustable by tap II, on the positive potential source I4. The direct current potential of point S is less than that of the cathode connection point Y by lan amount Eg, andY therefore the diode rectier of control unit K will not develop a direct current output until the peak input Voltage E1 exceeds the bias Voltage Eg.

The output circuit of the demodulator includes a radio frequency choke L1 and an output resistance R3, the low potential terminal of the output resistance being grounded through an audio frequency by-pass condenser I8. To suppress the audio output for weak input voltages, a delay bias voltage is provided between the cathode C and the grid G1 of the demodulator and this bias voltage is controlled automatically by the control unit K. The direct current potential developed across resistance R2 by the diode rectifier C-G2 is impressed upon the control grid of a direct current amplifier I9 which has output circuit resistors R4, R5 in the plate and screen grid circuits, respectively. Positive potentials are impressed upon the plate and screen grid by a lead 20 which connects the junction of the resistances R4, R5 to the point Y on the direct current source I4. The low potential terminal of the audio output resistance Ra is connected to the plate of tube I9 through a filter resistance 2|.

Atap 22 on the audio output resistor R3 permits any desired portion of the available audio output .to be impressed upon the amplifier tube 23 of the succeeding unit G of the system. The cathode of the amplifier tube is connected by lead 24 to a point on the direct current source I4- which is at a greater positive potential than that of the point Y and the grid of the tube is connected, through a filter comprising resistances 25 and condenser 26, to the screen grid of the direct current amplier I 9. A blocking condenser 21 is included between tap 22 and the grid to isolate the grid from the direct current potential of the resistance R3.

Audio frequency by-pass condensers 28 are provided between the anodes of the two control unit rectifiers and the anode G1 of the audio frequency rectifier.

In the absence of a rado frequency input at theA antenna, the plate current flow in the direct current amplifier I9 is high and the delay bias voltage on grid G1 is substantially the same as the delay bias voltage Eg. So long as the peak input voltage E1 to tube I0 is less than the delay bias Eg on the grid G2, this condition obtains and the high bias on the demodulator renders the transmission system inoperative beyond the point I of the schematic system of Fig. 1, i. e., the tube I0 of Fig. 2. For peak values of E1 less than the delay bias Ep on the gain control unit J, there will be no reduction in the gain of the radio frequency amplifier system preceding the point I. For values of E1 greater than Eg but less than Ep, a direct current potential is developed across the resistance R2 of the control unit K, thus reducing the plate current flow in the resistor R4 of tube I9 which reduces the delay bias between the demodulator cathode C and grid G1. This permits the transmission system to regain its normal operation beyond the point I or tube IU and up to the blocking condenser 21.

The audio transmission is also blocked, for Weak signals, by the high negative bias established on thegrid of the amplifier tube 23. As the value of the input Voltage E1 increases in the range between Eg andrEp, the reduction of current flow in the screen grid system of the direct current amplifier I9 decreases the bias on tube 23 to permit its operation as an audio amplifier. The circuit elements comprising the control unit K therefore suppress Vall audio transmision through the system for weak signals but restore the normal transmission characteristics for rectierinput voltages E1 above a predetermined value Eg. Adjustment of the point S along the resistor I4 makes possible a selection of the carrier level at the antenna below which no receiver output is obtained.

The effect of the two automatic control units on the transmission through'the receiver system will be apparent from the several curves of Fig. 3, which curves shown the relationship between antenna input and audio output, at reproducer I, for several values of delay bias voltage Eg. In obtaining the data for these curves, the carrier modulation was set at 10% and the tap I'I was adjusted along the direct current source I4 to vary the delay bias Eg on the rectifier `element of unit K between zero and a maximum of 40 volts, the bias for each curve being indicated by the legends such as Eg=10 appearing adjacent the curves. The delay bias on the automatic gain control unit J was such that no gain control voltage was developed for antenna' input voltages less than about 15 microvolts.

The typical output-input characteristics of previous types of sensitive automatic gain control receivers are Well indicated by the curve for zero transmission suppression bias. It will be noted that the audio output does not fall 01T uniformly as the antenna input decreases below the critical value at which the automatic gain control comes into operation. In the range of very Weak signals, the total audio output decreases at a relatively low rate, the greater part of the output being due to fluctuation noises arising within the receiver. Reception over the range of antenna voltages up to about 10 microvolts is not satisfactory due to the noise component and, over the greater part of that range, it is not possible to distinguish the signal output as it is completely masked by the noise output.

The total audio output in this range of signals is reduced when the delay bias Eg on the control unit K is adjusted to suppress transmission for very weak signals. The curve for Eg=l0 volts shows that the audio output decreased rapidly, With decreasing signal strength, for carrier inputs of less than 4 microvolts. Increasing the delay bias Eg to l5 volts provided a much sharper cutoff for inputs below '7 microvolts, the remaining curves showing sharp cutofs at higher carrier inputs. The curve for the value of Eg=30 volts indicates that all transmission beyond the control point was suppressed until the automatic gain control through the unit J was fully established, i. e., substantially constant audio output is obtained for all ranges of antenna input voltages above that critical value corresponding to a rectifier input voltage E1 suicient to initiate the removal of the blocking action of the control unit.

An examination of the curves for delay bias voltages of 30, 35 and 40 volts will show that, in terms of increasing antenna input, the effect of an increase of the delay bias from 35 to 40 volts has more effect than an increase from 30 to 35 volts. This results from the fact that the automatic gain control unit J is functioning, in the upper half of this range, to prevent the rectifier input E1 from increasing at as rapid a rate as the increase in the antenna input.

The several curves illustrate the varying transmission characteristics which may be obtained by adjustment of the critical input voltage below which the audio output is reduced or eliminated. The appropriate adjustment for any given receiver will depend upon the absolute magnitude of the iiuctuation noises arising in the receiver and the noises arising from outside disturbances. With the receiver for which the curves of Fig. 3 were plotted, a delay bias Eg of l5 volts was suincient to suppress receiver noises and the suppression of additional noises due to local disturbances may be effected by increasing Eg to 30 volts or 35 volts. i

It will be apparent that embodiments of an automatic transmission control system such as shown in Fig. l may take other forms than that shown in the circuit diagram oi Fig. 2. As stated above, either or both control units may employ amplication before and/ or after rectification. As shown in Fig. 2, the unit K includes the recti- I'ler C-Gz and the direct current amplier but it will be apparent to those familiar with the design of wave transmission circuits that the tube I9 may be made a rectifier by appropriate changes in the circuit connections.

In its broader aspects, the invention comprises the addition to a transmission system of two gain control units which automatically determine the transmission through the system. With such control units, two'separate portions of the transmission system are normally biased, i. e., in the absence of a radio frequency input, in opposite senses, one portion being biased for maximum gain while a second portion is biased to suppress transmission. The control units are preferably connected across the system at a point intermediate these sections, and with increasing input voltages, the control units operate in opposite sense to reduce the normal gain of one portion of the system and to remove the blocking action normally impressed on the other section. The net eiTect of the two controls is to maintain a substantially constant output for all signals of an order sumcient to produce a satisfactory output, and to eliminate all output for signals of lesser magnitude.

Itis to be understood that there is a considerable latitude in the design of the control units and, obviously, in the types of transmission systems in which such units are incorporated.

I claim:

i. In a carrier Wave receiver, the combination with a diode demodulator, and means impressing a bias potential on said demodulator to prevent the development of an audio output voltage until the impressed carrier voltage exceeds a predetermined value, of means for removing said bias potential as said impressed carrier voltage increases above said predetermined value and additional means for adjusting said value.

2. In an electrical wave transmission system, a double diode rectier having a cathode and two anodes, a single input impedance for said rectifier, circuit elements connected to the respective anodes to constitute substantially independent audio frequency vand direct currentv output circuits, means for impressingy a negative poten.-

tial on one of said anodes to prevent the development of current flow in the output circuit 'thereof until the impressed radio voltage exceeds a predetermined magnitude, and means auomatically varying said impressed negative potential with variations in the radio voltage impressed on said rectifier and means for selecting the radio voltage value at which said last means is operative.

3. In a transmission system, the combination with a demodulator having an anode, of means for impressing on said anode a negativedirect current. potential, of means automatically reducingsaid impressed potential. to a negligible value when the received .signal energy exceeds a predetermined level and additional means for select.-

. ingthe signal energy level .at which said automatic means becomes operative..

4. In. a radio. receiver, the combination with. a radioy frequency amplier, andan audio frequency amplifier, of a demodulator connected between said amplifiers, and means including a pairof control units for automatically controllingthetransmission through the respective amplifiers in opposite sense and. asa function of the received radio energy and one of said control units having means to control the demodulator operation.

5. The invention asset forth in claim 4, wherein said controlunits eachzinclude a rectifier, and one of saidrectifiers ris in parallel with said demodulator.

6. A radio receiver of thetype including a radio. frequency amplifier, a detector, control means operative vautomatically to adjust the gain ofthe amplifier when the received. radio fre- ;quency voltage rises above a critical value, means normally. biasing. said detector to suppress all audiooutput, and;control means independent-of the action of the iirstcontrormeans operative.:

automatically to remove the suppression bias on said detector when the received radio frequency voltagefexceeds a critical value, characterized by meansv for adjusting .the critical voltage value at.

which one of said` control means becomes operative independently of the critical voltage value 1 of the second controlmeans.

,1. quency amplifier to render the same inoperative,

and means operative to remove said normalbias when .the received radio frequency voltage rises above a critical value, of meansfor adjusting the critical voltage value at4 which one control means .ibecomes operative independently of the critical voltage value of the. othercontrol means.

9. The invention as set forth'in claim 8,. wherein said. adjusting means regulates. the4 critical. voltage value at which said second control means Orbecomes operative toremove the normal biasv on said amplifier.

10. In a radio receiver, the` combination With.

a radio amplifier, of transmission control means comprising .a tube-having a cathode cooperating.

Gtwith a pair of anodes to function as two diodes,

input circuit means impressing uponeach diodea radio voltage developed by said radio amplifier,` an output resistance for each diode, and means impressing upon the first of said diodes a bias `voltagetending to prevent recticationthereby;

said biasingmeans comprising a biasing resistance between the cathode and the anode element of the said-rst diode, andmeans forxpassing through saidbiasing resistancea direct current ,which varies -as a .function .of the. rectified` direct current. .voltage .developed by? the: second dioder across its. associated output resistance and means for .preselecting the radio voltage-level'at. which said seconddiode isabletorectify.

11..A= radio :receiver as claimed in'claim 10, wherein .said last means comprises a second tube having a cathode cooperating with a control grid and.ianode,-and means .impressing between the said control grid and cathode the direct current.

voltage developed across the output resistance of the second diode, the said biasing resistance being connected between'the cathode and anode of the second tube.

12. Aradioreceiver as claimed in claim 10,1.'

wherein saidlast means includes a direct current amplifier, and mea-nsimpressing upon the same the direct current .voltage developed by said second diode, thesaid biasing resistance being ineluded inthe output circuit of said direct `current amplifier.

13. In. a-Lradio receiver, a. radio. amplifier, a;

14. A radioireceiver `as claimed in claim 13, in.

combination vwith means adjustable to control'the threshold value below which said biasing means renders said demodulator diode inoperative.

15...*In ,a radio receiver, the combination with a radiofzfrequencyamplifier, a demodulator stage, and a .stage'of audio frequency amplification, means normally 'impressing a bias voltage on said amplifier to produce maximum gain, and means normally impressing upon:.one of said stages-a bias voltage of a magnitude eiiective to prevent transmission therethrough, of control means operating automatically-when the receiver input exceeds a predetermined value to maintainthe amplified Voltage level at a point in said amplifier at a substantially xed'value over a wide range of `receivedsignal voltages, andcontrol means operable automatically when the received radio frequency voltage rises to a critical value to remove .said transmission-preventing bias voltage, wherein each of said control means includes a rectifier and means impressing thereon a bias voltage which rendersl the rectifier :inoperative until the peak radio input to therectifier exceeds the bias voltage thereon,` the biasing means of the rectifier of usaidiirst control means impressing thereon a biasvoltage less than that impressed on the .other rectifier by itsbiasing means.

16. Ina radio receiver, the combination with a radio frequency amplifier, a diode detector anda diode rectifier vonlwhich radio frequency voltages are impressed .by said amplifier, and means including a resistance in the circuit of said detector for applying thereto a delay bias voltage to render the detector inoperative for. radio input voltages. below acriticalvalue, of means including said rectifier and a direct current amplifier for establishing a current flow in said resistance to reduce said applied .bias voltage when the radio input voltage on said rectifier exceeds a predeter. mined valuefin combination with a third rectifier for automatically controlling the gain-of saidam'e.

plier in. accordance with variations in the strength of the radio frequency voltage impressed on said receiver.

1'?. In a radio receiver, the combination with a radio frequency amplifier, a diode detector and a diode rectifier on which radio frequency voltages are impressed by said amplifier, and means including a resistance in the circuit of said detector for applying thereto a delay bias voltage to render the detector inoperative for radio input voltages below a critical value, of means including said rectifier and a direct current amplifier for establishing a current flow in'said resistance to reduce said applied bias voltage when the radio input voltage on said rectifier exceeds a predetermined value, in combination With an audio frequency amplifier Working out of said detector and means impressing thereon a bias voltage to render the same inoperative, and wherein said direct current amplifier automatically removes the bias voltage on said audio frequency amplifier when the radio input voltage on said receiver exceeds a predetermined value.

18. In a radio receiver, the combination with a radio frequency amplifier and an audio frequency amplifier of three rectiiers having their input terminals connected in parallel and across the output terminals of said radio frequency amplifier, one rectifier developing an audio frequency voltage for application to said audio amplifier, means energized by the direct current voltages developed by other rectiiiers for automatically controlling respectively the radio frequency and the audio frequency transmission through said receiver, said first rectifier being normally biased to prevent operation thereof, circuit elements cooperating with said means to render said first rectifier operative when the amplified radio frequency voltage impressed upon one of said other rectifiers reaches a predetermined value.

19. In a radio receiver, the combination with a diode demodulator, of means impressing a delay bias voltage on said demodulator, and means for automatically varying the said bias voltage as a function of the radio voltage input to the demodulator, said automatic means including a rectifier and direct current ampliiier for developing a direct current voltage of greater magnitude than that obtainable by rectification of the radio voltage on said demodulator, and means for adjusting the radio input level at which said rectifier is operative.

PAUL O. FARNHAM. 

